Naphtha steam-cracking quench



Aug- 30, 1960 R. H, JoHNsToN ETAL 2,951,029

NAPHTHA STEAM-CRACKING QuENcH Filed June 9, 1958 l N .l I w .n.ll,

Robert H. Johnston Earle A. Nrmaer Inventors NAPHTHA STEAM-CRACKING QUENCH Robert H. Johnston, Whippany, and Earle A. Nirmaier,

Maplewood, NJ., assignors to Esso Research and Engrneering Company, acorporation of Delaware Filed June 9, 1958, Ser. No. 740,764

11 Claims. (Cl. 20S- 100) This invention relates to a method ofquenching hot gaseous eluent from steam-cracked naphtha productstransferred from a cracking coil outlet into a quench tower forseparation of the products into fractions. It deals wtih a simplified,eiiicient and economical method of quenching the cracked products asthey are transferred to the quench tower and operating the quench towerfor quick separation of gases and naphtha vapor from a residualfraction.

In steam-cracking virgin naphtha fractions to produce high yields of C2to C5 olens and diolens using cracking conditions of high temperaturesand low pressures, quick quenching and fast separation of products arenecessary. The high temperatures and the high reactivity of the crackedproducts leaving the cracking coil are factors in the tendency towardpolymer tar and coke formation.

Minimizing formation of the tar and coke to increase the yields of thelow boiling oleiinic hydrocarbons is an object of the present invention.Attainment of this object involves the use of a quench oil havingsuitable boiling characteristics and refracton'uess to cracking underthe quenching conditions, and the use of large quantities of the quenchoil cooled and circulated to appropriate points in the quenching system.

Simultaneously with the quenching, it is important to er'ect a rapidseparation of the volatile components to be recovered, especially thevolatile and valuable C2 to C5 oleiins and diolens. For this reason, thequench tower is not constructed in the manner of conventionalfractionating towers which have a large number of plates that causeholdup in the flow of liquid and vapors.

The manner of quickly quenching the high temperature steam-crackednaphtha products and obtaining the quick separation of these productsinto suitable fractions will be described with reference to theaccompanying drawing.

In the drawing is shown a diagrammatic flow plan of means and stepsfound satisfactory for accomplishing the objects of the invention.

Referring to the drawing, a narrow cut virgin naphtha feedstock for thecracking starts from supply drum ll and is passed by line 2 into acracking coil located within the cracking furnace 3, where the crackingcoil is exposed to high intensity radiant heat. The preferred feedstockfor cracking is a naphtha containing principally a C5 or C6 to C10saturated aliphatic hydrocarbons, i.e., parafns and naphthenes, boilingprincipally in the range of 70 F. to 220 F. The feedstock may have asomewhat narrower or broader boiling range, e.g., in the range of 0 to330 F.

A suitable proportion of steam is added to the hydrocarbon fed from line4, generally to make the resulting cracking mixture contain 60 to 90mole percent steam, thus lowering the partial pressure of thehydrocarbons A to substantially less than half the total pressure. Inthe cracking coil located within the furnace 3, the naphtha hydrocarbonsmixed in vapor phase with steam are i 2,951,029 IC Patented Aug. 30,1960 heated in the range of 1350 to 1500" F., preferably above l400 F.The total pressure of the cracked reaction mixture is -in the range of lto 5 atmospheres and preferably less than 30 pounds per square inchabsolute. The cracked reaction mixture of steam and hydrocarbons passesthrough the cracking coil in furnace 3 in a fraction of a minute, thenon leaving the outlet of the coil is transferred by transfer line 5 tothe quench tower 6. A sufficient amount of relatively cool quench oil isinjected from line 7 into transfer line 5 at one or more points toeffect a rapid lowering of temperature in the cracked reaction products.This transfer line cooling from a coil outlet temperature that is above1350o F. to a temperature in the range of 450 to 650 lF. is effected ina few seconds in the stream passing through the transfer line. Y

The mixture of the cracked products and quench oil is preferablyintroduced to the bottom part of the quench tower at a temperature below650 F. and preferably close to 500 F. The quench oil ow rate from line 7into transfer line 5 has to be high with reference to the flow rate ofthe cracking coil efuent, e.g., from 3 to 6 parts by weight of quenchoil per one part by weight of cracking coil efuent, the quench oilhaving a temperature in the range of 300 to 400 F.

To prevent excessive coking in the transfer line 5, the quench oilsupplied from line 7 mustV be controlled in its polymer tar content,i.e., its content of materials which have a coke forming tendency. Inusing as a large proportion of the quench oil residual oil separated asbottoms in the quench tower 6, it is necessary to control the content ofpolymer tar by replacing a portion of the residual with an oil of lowerpolymer tar-forming reactivity, eg., refractory oil from an extraneoussource. This makeup oil may be added through line 8 to the quench oilline 7, but it is advantageously introduced into the residual oilrecirculated to the quench tower 6 as by line 9.

A good refractory quench makeup oil is obtained by light cracking of gasoil, for example, a catalytic cracking or other kind of low temperaturecracking, i.e., lower than used in steam-cracking. This oil is adistillate oil and preferably has an initial boiling point sub-v is 500or 525 to 660 F. The suitable gas oil makeupV is stable in being lessreactive under the quenching con-` ditions than oil derived from thesteam cracking.

When the quench makeup oil is added to recycled residual oil from quenchtower 6 owing through line 9, it becomes mixed with residual oil which,is circulated.

to an upper part of the quench tower 6. The residual fraction withdrawnas bottoms from the quench tower 6 at a temperature of 450 to 600 F.through line 10 is passed by pump 11 through line 12 for cooling in heatexchanger 13. The partly cooled oil leaving the heat exchanger 13 at atemperature in the range of 300 to 400 F. through line 14 becomesdivided into at least four streams. One stream of the partly cooledquench tower `bottoms is passed through line 43 to a mid-portion oftower 6 for partial cooling of vapors flowing upwardly through tower 6.A purge stream of the quench tower bottoms is withdrawn from line 15,which is a continuation of line 14. A purge stream `is withdrawn throughline 16 and heat exchanger 17.

The transfer line quench oil stream is withdrawn from line 15 throughline 7 which sends the quench oil into.

the transfer' line 5 at one `or more points, as previously mentioned. Aportion of the quench oil may be clay treated to remove polymer tars andthen returned into circulation as quench oil. The remaining partlycooled residual oil continues in line to be passed through heat exchangecooler 1S for cooling, and from there through line 19. Quench oilmakeup, e.g., gas oil, is introduced into line 19 from line 9.

The quench oil flowing through line 19 is divided into two or morestreams for particular purposes which will be further indicated. Onepart of the stream flowing through line 19 is diverted through line 20to an absorption tower for use as a lean sponge oil which absorbshydrocarbons higher boiling than ethene from an ethenerich gas stream inprocessing the light ends. The resulting fat sponge oil containingabsorbed hydrocarbons is returned through line 21 to be passed by line22 into the quench tower 6 at a point intermediate the top of the towerand the point in which the stream from line `43 enters tower 6, and theabsorbed hydrocarbons are released in said quench tower from said fatsponge oil. Valved connecting line 23 is useful for proportioning theamount of oil which is admixed from line 19 with the sponge oil that isreturned to the quench tower. Another part of the main stream flowingthrough line 19 Vis passed through line 24 to a top part of the quenchtower 6. This top quench stream introduced from line 24 has therelatively lowest temperature.

As an additional control of the polymer tar content in the residual oilcirculated from the bottom of tower 6, a quantity of this oil iswithdrawn from line 12 through line 25 for a clay treatment whichremoves polymer oils and the treated oil is returned to line 14 via line26.

The interior of quench tower 6 is equipped with a few plates, 27, andwith baffles 28, which may be in the form of inverted V-shaped sheds,for obtaining contact between liquid and vapor but allowing for fastflow of materials.

The necessary cooling is obtained in the upper part of the quench tower6 by injecting each of the quench oil streams in suitable amounts and atproper temperatures at the several spaced points.

The temperature of the vapors at the top of the quench tower 6 is set toavoid condensation of water in the upper part of the tower. For the samereason, the quench oil ow in each of the several streams is controlledin temperature and flow rate to avoid water condensation in the upperpart of the tower.

The gaseous stream that reaches the top of the quench tower to be takenoverhead therefrom through line 29 contains steam-cracked hydrocarbonproducts boiling below 430 F., steam, and some hydrogen. This gaseousstream is passed via line 29 into cooling condenser 30 at a sufficientlylow temperature to condense out water and hydrocarbons having more than'live carbon atoms per molecule. The condensate is collected in theseparating tank 31, wherein the liquid condensate is settled so that -alower water layer can be withdrawn through line 32 and condensed oil canbe withdrawn from an upper liquid layer through line 33. Uncondensedgaseous hydrocarbon products containing principally oleiins anddioleiins having up to six carbon atoms per molecule are withdrawn fromvessel 31 through line 34 to be subjected to light ends processing. Thelight ends processing steps `are such as to recover ethene, propene,butenes, butadiene, etc.

As the present invention is principally concerned with the quenchingoperation, a more detailed explanation follows regarding the quench oilcircuit.

The total flow rate of the main quench oil stream, i.e., residual oilfrom the bottom of quench tower 6, can be controlled by the circulatingpump 11. Various temperature and pressure controlling means may be used.

Heat exchanger 13 lowers the temperature of the main quench oil streamfrom a temperature above 450 F., near 500 F., or higher to a temperaturein, tht? range in the range of 175 to 250 F. The heat exchangers extractlarge amounts of heat useful for producing steam.

A purged portion of the main quench stream withdrawnl through line 16 isgenerally l to l0 percent of the main quench stream. To replace thepurged amount, the fresh light gas oil quench makeup added through line9 is approximately similar in amount.

ri`he makeup quench oil may come from storage at a temperature about F.or lower. This makeup oil is preferably blended with the hotter quenchoil stream such as coming from the heat exchanger 13 so that thecombined stream liowing through line 19 has a temperature in the rangeof to 200 F., preferably close to F. Thus, a portion of the oil ilowingthrough line 19 is brought to a satisfactory temperature level forintroduction to the top of quench tower 6 and therein gives adequatecooling without causing condensation of water.

A major portion `of the stream flowing through line 19 is preferablypassed into the quench tower through line 22 at a temperature 5 to 50Fahrenheit degrees higher than the temperature of the top quench streamwhich flows through line 24. This intermediate stream will containsponge oil, such as returned through line 21, and preferably has atemperature in the range of 175 to 200 F.

Line 43 introduces a third quench oil stream and this stream has atemperature preferably in the range of 350 to 400 F.

A favorable manner of proportioning the different quench streams sentinto tower 6 is to have the inter mediate quench oil stream from line 22iiow at a rate approximately three times that of the upper and lowerquench streams sent into the upper part of tower 6, i.e., above thecracked product inlet. Altogether the total fiow of quench oil from thestreams to the mid-portion and upper part of quench tower 6 will amountto nearly half the flow rate of the main quench stream withdrawn fromthe bottom of tower 6, and this flow rate is in the order of 3 to 6times the flow rate of hydrocarbons entering the cracking coils.

In the quench tower 6, the cracked vapors entering from the transferline 5 pass up countercurrently to the successive quench oil washes tohave their temperature lowered at a fast rate, for example, lowered fromabove 450 F. to a temperature in the range of 220 to 275 F., preferablyclose to 225 F. when the pressure is slightly above one atmosphere. Apreferred mode of operation is illustrated fur-ther by the followingexample.

Example Various virgin naphtha fractions may be used as the feedstocksof the steam-cracking. A preferred feed contains hydrocarbons boilingprincipally in the range of 70 to 220 F. The feed is cracked attemperatures close to l400 F. in the presence of sufficient admixedsteam to make the hydrocarbon partial pressure near l2 pounds per squareinch absolute or less. The cracked products leaving the outlet coil at'nearly l1400o F. are then contacted lin a transfer line with quench oilhaving :a temperature of 375 F., the proportion of quench oil beingclose to four times the weight of the admixed cracked hydrocarbonproductsV which are quickly cooled to below 650 F. before they enter thebottom part of the quench tower.

The amount of quenchroil injected into the transfer line is madeadequate to bring about a lowering of the temperature of the crackedhydrocarbon products, to a asenoee S temperature of 500 F. to 525 F.,within shont periods, e.g., a fraction of a second up to about twoseconds.

The quench oil which is injected into the transfer line is made up ofresidual oil from the quench tower supplemented with the light gas oilof refractory character, c g., a catalytically cracked gas oil productboiling in the range of 525 to 660 F. A similarly useful supplementalquench oil may be obtained by chemical treatments of cracked gas oils toremove unsaturates. Various virgin light gas oil fractions may be used.Tests have indicated that the quench oil should not be made up only ofrecycled residual oil from the quench tower without the supplementallight gas oil if excessive cooking is to be avoided in the transfer.

In the combined quenching and fractionating zone of the quench tower 6,the gaseous products are cooled by being brought successively intocontact with the quench oil streams that have lower temperatures as theyare introduced at higher levels, the predominant amount of cooling beingdone by the quench oil introduced into the upper part of the tower attemperatures preferably in the range of 170 to 200 F.

With the type of system described, the quench tower can be used withseveral more than one cracking coil. The quench oil can be injected -atmore than one point at the outlet of the coil or in the transfer lines.

Several observations on effects and results are summarized as follows:

The use of residual oil with accumulated tarry polymer bottoms from thequench tower per se as a transfer line quench oil gives intolerable cokein the transfer line. Highly improved quenching from the Viewpoint oflow coke formation is obtained when a portion of the quench towerbottoms is purged and replaced by a makeup light gas oil that ispreferably refractory and has low coking tendencies.

The recovery of the desired olefins and diolens up to six carbon atomsper molecule is improved by having a fast ow of the steam with Volatilehydrocarbons up through the quench tower, using large amounts ofrelatively cool quench oil to cool the vapor mixture.

The amount of residual oil withdrawn from the quench tower, then cooledand circulated to` the upper part of the quench tower to act as acooling reflux, has to be substantially larger in liquid iiow rate lthanthe liquid flow rate of the initial cracking feed, for example, three orfour times greater on the basis of pounds per hour.

For the purpose of obtaining the desired high yields of ethylene,propylene, butylenes and butadienes, a narrow virgin naphtha fractioncontaining C or C6 to C8 or to C hydrocarbons is used `as the crackingfeed. As the cracking temperature is raised to above 1400 F., theproportion of oletins and dioleiins in the product increases.

The yield and quality of the desired products is also improved byimproving the rapidity of quenching in the transfer line and theseparation or" the desired gaseous products in the quench towerquenching and fractionating zone.

Modifications may be made which come within the scope lof the invention,as for example, a step of separating a heavy distillate fraction in abottom part of the quench tower for circulating as a quench oil. Thereare some disadvantages, however, in separating an intermediatedistillate in the quench tower, since such a separation causes liquidholdup and slows down the liow of vapor up through the tower. It is tobe noted that in using a bottoms residual in the quench circuit,satisfactory results are obtained by purging a portion of this bottomsand replacing the purged portion by a light gas oil.

The invention as described is claimed as follows:

l. Ina process of steam-cracking hydrocarbons at an elevated temperatureand low4 hydrocarbon partial pressure to form unsaturated C2 to C6hydrocarbons, the

improvement which comprises mixing with cracked products leaving acracking zone a quench oil in a sucient amount to rapidly lower thetemperature of the resulting mixture from above about 1350 F. to belowabout 650 F. as the mixture is passed to a quenching and fractionatingzone, separatingV from the quench cracked products in said quenching andfractionating zone a residual fraction boiling above about 430 F. andlower boiling vapors which pass rapidly upwardly through the quenchingand fractionating zone countercurrent to relatively cooler quench oil,said quench oil being mainly residual oil withdrawn from the quenchingand fractionating zone below the point at which said mixture is passedinto said quenching and fractionating zone, then cooled land a portionof the cooled quench oil circulated to the cracked products leaving thecracking zone and another portion of cooled quench oil passed to anupper part of said quenching and fractionating zone, athird portion ofthe thus circulated quench oil being purged and replaced by a morestable gas oil which lowers the tar content of the quench oil.

2. In a process of steam-cracking a virgin naphtha fraction attemperatures in the range of 1350 to 1500" F. and low hydrocarbonpartial pressures, the improvement which comprisesy admixing cooledquench oil with 4steam-cracked product -being transferred in a transferline from a cracking zone to a quench tower in which cracked vaporproducts boiling below 430 F. are cooled by a dofwnflowing cooled quenchoil and are separated from higher boiling components that remain withthe quench oil as ak residualoil; withdrawing quench oil containing saidresidual oil from the bottom part of said quench tower; cooling,circulating and using a iirst portion of the withdrawn quench oil yasthe cooled quench oil `admix'ed in said transfer line; further coolingand circulating a second portion lof the withdrawn quench oil to thequench tower to form said downflowing cooled quench oil; purging a thirdportion of the withdrawn quench oil; and replacing the purged portion ofthe withdrawn quench `oil with oil of relatively lower polymertarforming reactivity, thus giving the circulated withdrawn quench oil alower tar content.

3. In a process as defined in claim 2, said oil of relatively lowpolymer-tar forming reactivity being a refractory gasoil fraction formedby cracking at lower temperatures than -those used in thesteam-cracking.

4. In a process as defined by claim 2, said quench oil passed to thetransfer line being injected at more than one point into said line.V

5. In a process as dened by claim 2, said quench oil pumped back from abottom part ofthe quench tower to its upper part being from three to sixtimes the weight of cracked hydrocarbons entering the tower.

6. In a process of steam-cracking `a virgin naphtha fraction attemperatures in the range of 1350" to l500 F. and low hydrocarbonpartial pressures, the improvement which comprises admixing cooledquench oil with steam-cracked product being transferred in a transferline from a cracking zone to a quench tower in which cracked vaporproducts boiling below 430j F. are cooled by `a downflowing cooledquench oil and are separated from higher boiling components that remainwith the quench oil as a residual oil; withdrawing quench oil containingsaid residual oil from the bottom part of said quench tower; treating atleast al part of said withdrawn quench foil with clay to remove polymertars and returning said part of withdrawn quench oil into circulation ascooled quench oil; cooling, circulating and using ,a iirst portion ofthe withdrawn quench `oil as the cooled quench oil admixed in saidtransfer line; further coolingand circulating a second portion of thewithdrawn quench oil to the quench tower to form said downiiowing cooledquench oil;v purging ka third portion of the withdrawn quench oil andreplacing the purged portion of the withdrawn quench oil with oil ofrelatively lower polymer- 7 tar forming reactivity, thus giving thecirculated withdrawn quench oil a lower tar content.

7. In a process of steam-cracking a virgin naphtha fraction attemperatures in the range .of 1350 to1500 F. and `low hydrocarbonpartial pressures, the improvement which comprises admxing cooled quenchoil-with steam-cracked product being transferred in a transfer line froma cracking zone to a quench tower in which cracked vapor productsboiling below 430 F. are cooled by a downiiowing cooled quench oil andare separated from higher boiling components that remain with the quenchoil as a residual oil; withdrawing quench oil containing said residualoil from the bottom part of said quench tower; cooling, circulating andusing a first portion of the withdrawn quench oil as the cooled quenchoil admixed in said transfer line; further cooling and circulating asecond portion of the withdrawn quench oil to the quench tower to formsaid downfiowing cooled quench oil; said further cooled quench oil beingat a temperature in the range of about 150 to 200 F. and beingcirculated to an upper part of the quench tower at a rate to preventcondensation of water vapor in said upper part of said quench tower;purging a third portion of the withdrawn quench oil and replacing thepurged portion of the withdrawn quench oil with oil of relatively lowerpolymer-tar forming reactivity, thus giving the circulated withdrawnquench oil a lower tar content.

8. In a process of steam-cracking a virgin naphtha fraction attemperatures in the range of 1350 to 1500 F. and low hydrocarbon partialpressures, the improvement which comprises admixing cooled quench oilwithy steam-cracked product being transferred in a transfer line from acracking zone to a quench tower in which cracked vapor products boilingbelow 430 F. are cooled by a downflowing cooled quench oil and areseparated from higher boiling components that remain with the quench oilas a residual oil; withdrawing quench oil containing said residual oilfrom the bottom part of said quench tower; cooling, circulating andusing a first portion of the withdrawn quench oil as the cooled quenchoil admixed in said transfer line; further cooling and circulating asecond portion of the withdrawn quench oil to the quench tower to formsaid downflowing cooled quench oil; said second portion of the withdrawnquench oil being circulated to said quench tower being divided into twostreams, one stream being circulated to the top part of the quench towerand the other stream being circulated to a place intermediate a middleportion of the quench tower and the top of the quench tower at atemperature higher than that of the stream circulated to the top part ofthe quench tower; purging a third portion of the withdrawn quench oiland replacing the purged portion of vthe withdrawn quench oil with oilof relatively lower polymer-tar forming reactivity, thus giving thecirculated withdrawn quench oil a lower tar content.

9. In a process of steam-cracking a Virgin naphtha fraction attemperatures in the range of 13.50 to l500 F. and low hydrocarbonpartial pressures, the improvement which comprises admixing cooledquench oil with steam-cracked product being transferred in a transferline from a cracking zone to a quench tower in which cracked vaporproducts boiling below 430 F. are cooled by a downflowing cooled quenchoil and are separated from higher boiling components that remain withthe quench oil as a residual oil; withdrawing quench oil containing saidresidual oil from the bottom part of said quench tower; cooling,circulating and using a iirst portion of the withdrawn quench oil as thecooled quench oil ad-V mixed in said transfer line; further cooling andcirculating a second portion of the withdrawn quench oil containingresidual oil to the quench tower to form said downowing cooled quenchoil; said second portion of withdrawn quench oil being divided into twostreams, one stream at a temperature of about k350" to 400 F. beingcirculated to a middle portion of the quench tower and another stream ata temperature of about to 200 F. being circulated to an upper part ofthe quench tower; purging a third portion of the withdrawn quench oil;and replacing the purged portion of the withdrawn quench oil with oil ofrelatively lower polymer-tar forming reactivity, -thus giving thecirculated withdrawn quench oil a lower tar content.

10. In a process of steam-cracking a virgin naphtha fraction attemperatures in the range of 1350 to 1500 F. and low hydrocarbon partialpressures, the improvement which comprises admixing cooled quench oilwith steam-cracked product being transferred in a transfer line from acracking zone to a quench tower in which cracked vapor products boilingbelow 430 1F. are cooled =by a downflowing cooled quenchoil and areseparated from higher boiling components that remain with the quench oilas a residual oil; withdrawing quench oil containing said residual oilfrom the bottom part of said quench tower; cooling, circulating andusing a irst portion of the withdrawn quench oil containing residual oilas the cooled quench oil admixed in said transfer line; further coolingand circulating a second portion :of the withdrawn quench oil containingresidual oil to the quench tower to form said downflowing cooled quenchoil; a part of said second portion of withdrawn quench oil being used asa sponge oil absorbent to absorb hydrocarbons boiling higher thanethene, then being circulated with another part of said second portionof withdrawn quench oil to the upper part of the quench tower where theabsorbed light hydrocarbons are released; purging a third portion of thewithdrawn quench oil and replacing the purged portion of the withdrawnquench oil with oil of relatively lower polymer-tar forming reactivity,thus giving the circulated withdrawn quench oil a lower tar content.

1l. In a process of steam-cracking a virgin naphtha fraction attemperatures in the range of 1350 to l500 F. and low hydrocarbon partialpressures, the improvement which comprises admiring cooled quench oilwith steam-cracked product being transferred in a transfer line from acracking zone to a quench tower in which cracked vapor products boilingbelow 430 F. are cooled by a downflowing cooled quench oil and areseparated from higher boiling components that remain with the quench oilas a residual oil; withdrawing quench oil containing said residual oilfrom 'the bottom part of said quench tower; cooling, circulating andusing a first portion of the withdrawn quench 'oil as the cooled quenchoil admixed in said transfer line; further cooling and circulating asecond portion of the withdrawn quench oil to the quench tower to formsaid downflowing cooled quench oil; said second portion of the withdrawnquench oil being divided into three streams, two streams being attemperatures of about to 200 F. and being circulated to the upper partof said quench tower and a third stream being at a temperature of about350 to 400 F. and being circulated to a middle portion of the quenchtower; purging a third portion of the withdrawn quench oil containingresidual oil; and replacing the purged portion of the withdrawn quenchoil with oil of relatively lower polymer-tar forming reactivity, thusgiving the circulated r I u w1thdrawn quench oil a lower tar content.

References Cited in the tile of this patent UNITED STATES PATENTS2,358,912 Dimmig Sept. 26, 1944 2,363,903 Smith Nov. 28, 1944 2,366,521Guichet Jan. 2, 1945 2,672,489 Holland Mar. 16, 1954 2,736,685 Wilson etal. Feb. 28, 1956

1. IN A PROCESS OF STEAM-CRACKING HYDROCARBONS AT AN ELEVATEDTEMPERATURE AND LOW HYDROCARBON PARTIAL PRESSURE TO FORM UNSATURATED C2TO C6 HYDROCARBONS, THE IMPROVEMENT WHICH COMPRISES MIXING WITH CRACKEDPRODUCTS LEAVING A CRACKING ZONE A QUENCH OIL IN A SUFFICIENT AMOUNT TORAPIDLY LOWER THE TEMPERATURE OF THE RESULTING MIXTURE FROM ABOVE ABOUT1350*F. TO BELOW ABOUT 650*F. AS THE MIXTURE IS PASSED TO A QUENCHINGAND FRACTIONATING ZONE, SEPARATING FROM THE QUENCH CRACKED PRODUCTS INSAID QUENCHING AND FRACTIONATING ZONE A RESIDUSAL FRACTION BOILING ABOVEABOUT 430*F. AND LOWER BOILING VAPORS WHICH PASS RAPIDLY UPWARDLYTHROUGH THE QUENCHING AND FRACTIONATING ZONE COUNTERCURRENT TORELATIVELY COOLER QUENCH OIL, SAID QUENCH OIL BEING MAINLY RESIDUAL OILWITHDRAWN FROM THE QUENCHING AND FRACTIONATING ZONE BELOW THE POINT ATWHICH SAID MIXTURE IS PASSED INTO SAID QUENCHING AND FRACTIONATING ZONE,THEN COOLED AND A PORTION OF THE COOLED QUENCH OIL CIRCULATED TO THECRACKED PRODUCTS LEAVING THE CRACKING ZONE AND ANOTHER PORTION OF COOLEDQUENCH OIL PASSED TO AN UPPER PART OF SAID QUENCHING AND FRACTIONATINGZONE, A THIRD PORTION OF THE THUS CIRCULATED QUENCH OIL BEING PURGED ANDREPLACED BY A MORE STABLE GAS OIL WHICH LOWERS THE TAR CONTENT OF THEQUENCH OIL.